The coasting time of a turbine, for example a gas or steam turbine, is defined as the time between shutting off the turbine drive system and reaching a lower limit value of the turbine speed. It is a function of the mechanical state of the turbine and is influenced for example by the state of the bearings and their supply of oil or the ventilation or possible contact with the turbine blades.
The coasting time of a turbine can therefore be used to monitor the state of the turbine and thus as an indicator of machine problems. It is possible in this manner to identify and eliminate technical problems at an early stage, before permanent damage occurs.
The coasting time of turbines is therefore determined manually or automatically and compared with a reference value. Deviations between the measured coasting time and the reference value may indicate machine problems and must be investigated further. Of course the coasting time of a turbine is also influenced by the current temperature of the medium in the turbine (intake air in gas turbines and respectively water vapor in steam turbines). However this temperature is subject to natural or operation-induced fluctuations, which can result in fluctuations of around 5% to 10% in the measured coasting time. A fluctuation range of 5% to 10% is however too large to permit a meaningful comparison with the reference value and therefore sensitive monitoring of the coasting process.